Recently, there has been a rise in prices of petroleum resources. Accordingly, there have been a number of studies on processes for collecting oils from oil sand layers buried underground in areas such as Canada or Venezuela. The oil sand is generally in a layer having a thickness of about 50 m. This layer is underground at a depth of several hundred meters. However, oils in the oil sand cannot be collected by extracting them at a normal temperature due to their high viscosity. Therefore, at the present time, one method of extracting the oils comprises raising the temperature of the oils by injecting heated steam into the oil sand layer. This reduces the viscosity of the oils. However, this method is not efficient due to the increase in cost. Accordingly, studies have been carried out to provide a highly productive method of collecting oils. One such method involves the use of two conduct pipes (steel pipes or stainless steel pipes) each having an electrode part on the end thereof. The pipes are buried at an interval of about 30 to 200 m so that the electrode part is situated on the oil sand layer. Then an electric potential of several hundred volts to several ten thousand volts is applied to both electrodes. This raises the temperature of the oil sand layer by Joule's heat and reduces the viscosity of the oils.
The specific resistance of the oil sand layer is several times higher than that of the upper strata. Accordingly, it is necessary for the conduct pipes to be covered with an electrically insulating material over the part which is buried in the strata in order to prevent the electric current from flowing through the upper strata. If the conduct pipes are not covered with an electrically insulating material, the electrical current does not flow between the electrodes buried in the oil sand layer, because it flows through the stratum part. Accordingly, there is a need to develop conduct pipes covered with an electrical insulating material which can be used under such specific conditions.
The electrical insulating material should include the following characteristics:
(A) capable of withstanding voltage characteristic of several hundred volts to several thousand volts and a volume resistivity of at least 10.sup.6 .OMEGA.-cm at not only a normal temperature but also a temperature capable of reducing the viscosity of oils in the oil sand layer (about 300.degree. C.),
(B) capable of withstanding hot water having a temperature of about 300.degree. C., because water contained in the oil sand layer is heated to a temperature capable of reducing the viscosity of the oil sand layer (about 300.degree. C.), and
(C) a mechanical strength capable of attaching the electrode and mechanical impact strength of such a degree that it is not broken by contacting the wall of the pit when burying the electrode attached on the end of the conduct pipe in the oil sand layer.
Thermoplastic resins such as polyethylene, polypropylene, polybutylene and polystyrene are known as materials having excellent water resistance. However, they have poor heat resistance and therefore cannot be used as a water resistant material at temperatures above about 80.degree. C.
Thermoplastic resins such as polysulfone resin, polyether sulfone resin, polyphenylene sulfide resin and polytetrafluoroethylene resin have been developed and used as resin materials having improved hot water resistance. These thermoplastic resins have excellent heat resistance and can withstand hot water. However, they are not desirable because their mechanical strength deteriorates when they are left in hot water.